Gynostemma guangxiense is an important medicinal species distributed in Guangxi, China. In this study, we not only obtained the complete chloroplast (cp) genome sequence of G. Guangxiense by Illumina paired-end sequencing technology, but also analyzed its codon usage pattern using bioinformatics approaches. The cp genome of G. guangxiense was 157,785 bp with a pair of inverted repeat (IR) regions (26,288 bp) separated by one large single copy (LSC) region (86,702 bp) and one small single copy (SSC) region (18,507 bp). The whole genome contains 130 unique genes, of which 113 are unique, including 79 protein-coding genes, 30 tRNA genes, and 4 rRNA genes. In addition, 62 repeats and 70 simple sequence repeats (SSRs) were identified, phylogenetic inference based on 73 protein coding genes indicated that G. guangxiense had the closest relationship to G. caulopterum. 52 CDSs longer than 300 bp in G. guangxiense cp genome were screened to analyze synonymous codon usage. The neutrality plot reflected a weak correlation between GC₁₂ and GC₃; ENC plot analysis showed that a majority of genes distributed below the expected curve; PR2-plot mapping analysis revealed G and T were used more frequently than C and A at the third base position. Finally, a total of 16 codons were identified as the optimal codons. These findings suggested that selection was the major pressure to influence the codon usage in G. guangxiense cp genomes. Our study provided an important theoretical basis for molecular identification, utilization and conservation in G. guangxiense.

Introduction: Ecological stoichiometry assessment provides insights into the dynamic interactions between the leaf–litter–humus–soil (LLHS) in different forest types. However, research on ecological stoichiometry in mixed and pure forest ecosystems is relatively weak in the karst region of southwestern China. Methods: We investigated how stand types influence the stoichiometric characteristics of carbon (C), nitrogen (N), and phosphorus (P) across the LLHS continuum. Leaf, litter, humus, and soil samples were collected from an evergreen coniferous forest ( Cryptomeria japonica var. sinensis Miquel ; CJ), a deciduous broadleaf forest ( Liquidambar formosana Hance; LF), and a mixed forest composed of both species ( C. japonica var. sinensis × L. formosana ; CJ × LF) in the fragile karst ecosystem of Southwest China. Results: The results showed that stand type had a significant effect on the concentrations of C, N, and P and on the stoichiometric ratios of all LLHS components. Compared with the two pure forests, litter in the mixed forest exhibited significantly higher C/N and C/P ratios, while N and P concentrations were significantly lower. In the broadleaf forest, leaf C/P and N/P ratios were significantly higher than those in the mixed forest and C. japonica monocultures, whereas leaf P concentrations were significantly lower than C. japonica monocultures. Compared with the mixed and broadleaf forests, humus C/N and C/P ratios showed significantly higher in the coniferous forest, while N and P concentrations and the N/P ratio were significantly lower. In the 0–20 cm soil layer, mixed forests had significantly higher C concentration, C/N ratio, and C/P ratio than the two pure forests ( p < 0.05). The P nutrient resorption efficiency of plant nutrients in the study area (40.07%–53.69%) was higher than that of N (10.43%–27.04%). Leaf C, N, P, C/N, N/P correlated positively with litter ( p < 0.01); leaf N, P, C/N, C/P correlated negatively with humus. Leaf N, C/N, C/P linked positively, leaf P negatively with soil ( p < 0.01). The explanatory power of litter, humus, and soil for leaf nutrient concentrations and stoichiometric ratios decreased in the order litter (50.8%) > humus (32.2%) > soil (1.8%). Discussion: Overall, these results indicate that the selection of stand type and dominant tree species is central to forest restoration and management in fragile karst ecosystems and plays a crucial role in maintaining the dynamic balance of C, N, and P in forest ecosystems.